Hydraulic piston pump unit with integral fluid reservoir

ABSTRACT

Single and dual hydraulic piston pump units comprising at least one pump unit including a variable displacement axial piston main pump rotatably journalled within an at least one pump housing, and operatively interconnected for dual pump units, the pump housing(s) being of a size having an integral fluid reservoir for the at least one pump unit, the reservoir including a drain port and plug and the housing including an air venting device. A block portion includes at least one pair of reversible fluid inlet and outlet ports and a fluid filter connecting portion. At least one center through shaft drives the main pump(s), the charge pump, and a cooling fan. A single pump unit in combination with a rotatable electric motor comprises a portable hydraulic power-producing unit. A dual pump unit in combination with a vehicle, having independently hydraulic motor-driven left and right wheels, permits independent wheel propulsion thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/619,679, filed on Oct. 18, 2004, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention pertains to hydraulic piston pump units, specifically to both single and dual piston pump units, having at least one pump housing wherein the pump housing(s) are of a size sufficient to include an internal volume portion sufficiently large to serve as an integral hydraulic fluid reservoir for the pump unit(s).

2. Description of the Prior Art

Hydrostatic transmissions have many and varied uses, including aiding in the propelling of vehicles, such as mowing machines, for example, and offer stepless control of the machines' speeds. A typical hydrostatic transmission system includes a pump unit having a variable displacement main pump connected, in closed hydraulic circuit, with a fixed displacement hydraulic motor. For most applications, the main pump is driven by a prime mover, such as an internal combustion engine or a rotatable electric motor, at a predetermined speed and in a specific direction. Changing the displacement of the main pump will change the output flow rate thereof which, in turn, controls the rotational speed of the motor. Main pump outflow can be reversed, thus reversing the direction of rotation of the hydraulic motor. In a vehicle, the hydraulic motor typically is connected, either directly or through suitable gearing, to the vehicle's drive wheels or tracks, etc.

The closed hydraulic circuit includes a first conduit connecting the main pump outlet with the hydraulic motor inlet, with a second conduit connecting the motor outlet with the main pump inlet. Either of these noted conduits may be the high pressure line depending upon the direction of pump displacement from its neutral position. A charge pump is added to the hydraulic circuit in order to charge the closed-circuit with hydraulic fluid through suitable check valves, thus making up for possible lost hydraulic fluid due to internal leakage. Additional valves can also be added to the closed-circuit in a manner well known in the art. For example, high pressure relief valves, sometimes referred to as shock valves, can be added to protect the hydrostatic transmission from being overloaded during its operation. Bypass valves can be used to allow hydraulic fluid to be routed from one side of the transmission to the other side, without significant resistance, and hot-oil shuttle valves can be used, to reduce the loop temperature of the closed circuit, by connecting the low pressure side of the closed loop to a drain, thus allowing replenishment with fresh, cooled, replacement hydraulic fluid.

Currently, most zero turn radius (ZTR) vehicles use dual hydrostatic transmissions. Each transmission is independent of the other and is used to drive a differing one of the vehicle's left and right side drive wheels or tracks. Each one of the dual transmissions includes a variable displacement over-center main pump with the required control valves, a fixed displacement hydraulic motor, together with the necessary fittings, hoses and the like. A single separate or remote reservoir and a charge pump inlet filter are shared by the dual transmissions.

In order to reduce the cost of the several pumps and minimize the vehicle installation work, it would be ideal if two pump units could be combined into a single pump unit, use a common integral fluid reservoir, and share the same charge pump.

The patent literature includes a large number of patents pertaining to dual pump constructions, including the following examples:

U.S. Pat. No. 6,682,312, to Ward, discloses a design of an inline tandem pump wherein two pumps are linked together using a pump interface or plate. Two pump shafts thereof are coupled near the middle of the unit by a pump shaft coupler or spline connection. A charge pump is added to the rear side of the second pump. In order to supply charge flow to the first pump, an additional external hose/tube is required to link the charge pump outlet and a port in the end cap of the first pump.

U.S. Pat. No. 6,705,840 B1, to Hauser et al., in one embodiment, discloses a design that combines two pumps into a single unit with face-to-face orientation of the pump rotating groups. Each pump includes a dedicated charge pump that is attached at a corresponding end of the unit to supply charge flow to its adjacent pump. A right angle “T” type gear mechanism, with a pair of bevel gears, is required to drive the unit in the middle of the unit.

U.S. Pat. No. 6,487,856 B1, to Ohashi et al., is similar to previously-noted U.S. Pat. No. 6,705,840 B1, to Hauser et al., but uses piston pumps with a yoke-type swash block supported by two radial bearings. The input shaft is at one end of the unit and a charge pump is located at the opposite end of the unit. Similar to previously-noted U.S. Pat. No. 6,682,312 B1, to Ward, it requires an additional external hose/tube to link the charge pump outlet and the end cap of the first pump.

U.S. Pat. No. 6,672,843 B1, to Holder et al. and U.S. Pat. No. 6,425,244 B1, to Ohashi et al., are similar in that both patents pertain to side-by-side piston pumps. None of the aforementioned dual pump units utilize integral reservoirs and filters

BRIEF SUMMARY OF THE INVENTION

Accordingly, in order to overcome the deficiencies of the prior art devices, the present invention pertains to both single and dual variable displacement hydraulic piston pump units wherein the pump unit housings are of a size sufficient to include an internal volume portion that serves as an integral fluid reservoir for the pump units. In the dual piston pump units, the two pumps, are included in a single unit and the pump housings are operatively interconnected and serve as a single, integral reservoir for both pump units. The pump housings and central block are designed to hold large amounts of hydraulic fluid within the unit, thus functioning as an integral fluid reservoir. A replaceable cartridge type fluid filter is preferably fitted into the bottom of the pump unit and functions as the charge pump inlet filter to reduce possible contamination. Various versions of these piston pump units, in addition to the noted integral reservoir, include the noted filter, a vent plug, a drain port and its associated drain plug, central through shaft(s) and a cooling fan, all amenable to be of compact constructions that can be produced at low manufacturing cost.

Specifically, in terms of structure, this invention pertains to a variable displacement hydraulic piston pump unit comprising in combination (a) one of an over-center axial piston main pump, a bent axis piston main pump and a radial piston main pump and (b) the main pump being rotatably journalled within a pump unit housing, the housing being of a size sufficient to include an internal volume portion that serves as an integral fluid reservoir for the pump unit.

In one version, the pump unit housing further includes an air venting device.

In another version, the fluid reservoir includes a case drain port.

A further version includes a block portion operatively interconnected with and having one end thereof physically adjoining one end of the pump unit housing. The block portion further includes a pair of reversible hydraulic fluid inlet and outlet ports, an air venting device, and a case drain port, as well as an air venting device. In this version, the block portion also includes an internal recess portion and a hydraulic fluid filter, with the filter being located within the center block internal recess portion.

Another version further includes an axial center through shaft. The through shaft extends axially through the main pump housing, the main pump, and the block portion. The through shaft includes an input portion, extending from another end of the pump unit housing remote from the block portion. The through shaft further includes another portion, extending from another end of the block portion remote from the pump housing, with the through shaft driving the main pump. This version further includes a charge pump. The charge pump has one end thereof adjoining the another end of the block portion. The charge pump is operatively interconnected with the block portion and driven by the another portion of the through shaft. Further included is a cooling fan. The cooling fan is axially spaced from the another end of the charge pump, remote from the block portion, and is driven by the another portion of the through shaft.

In a variation of the above version, the piston pump unit is in combination with a prime mover. The prime mover is operatively connected with the input portion of the through shaft for driving the through shaft and consequently the main pump. The pump unit, in combination with the prime mover, comprises a portable hydraulic power-producing unit.

A differing version includes a hydraulic fluid filter in operative combination with the block portion, with the hydraulic fluid filter being of the spin-on cartridge type.

Another embodiment of this invention pertains to a dual variable displacement hydraulic piston pump unit comprising in combination; (a) a first pump unit including a variable displacement axial piston main pump rotatably journalled within a first pump unit housing, the first main pump adjoining; (b) a second pump unit including a variable displacement axial piston pump rotatably journalled within a second pump unit housing; and (c) the first and second pump unit housings being operatively interconnected and of a size having an internal volume portion sufficient to serve as an integral hydraulic fluid reservoir for the first and second pump units. At least one of the pump unit housings includes an air venting device and the fluid reservoir includes a case drain port.

A further version includes a center block portion operatively interconnected, on each opposed axial end, with an adjoining first end of each of the first and second pump unit housings. The center block portion includes a pair of independent reversible hydraulic fluid inlet and outlet ports for each of the first and second pump units. At least one of the pump unit housings includes an air venting device and the fluid reservoir includes a case drain port.

In another version, at least one of the pump unit housings includes an air venting device and the center block portion includes an internal recess portion. A hydraulic fluid filter is located within the center block internal recess portion.

In a differing version, the first and second pump units further include a respective first and second axial center through shaft. The first and second through shafts extend axially through their respective main pump housing, main pump, and block portion. One of the first and second through shafts includes an input portion, extending from a first another side of its respective pump unit housing remote from the center block portion, with the first and second through shafts driving their respective main pumps. In a variation thereof, each of the first and second axial center through shafts further includes an inner end portion. The inner end portions extend into and operatively interconnect with each other within the center block portion. This version further includes a charge pump. The charge pump has one end adjoining the second another side of the respective pump housing. The charge pump is driven by the another portion of one of the first and second through shafts. This version additionally includes a cooling fan. The cooling fan is axially spaced from the another end of the charge pump. The cooling fan is driven by the another portion of one of the first and second through shafts.

In one version of this dual piston pump unit, the first and second pump units are arranged in an inline or tandem configuration, wherein the inline configuration is in a back-to-back arrangement.

In another version, the first and second pumps units are arranged in an inline back-to-back configuration with the center block portion being interposed therebetween.

In yet another version, the first and second pump units are arranged in a side-by-side configuration.

In a differing version, the first and second pump units are angularly offset relative to each other. In a variation thereof, the angular offset occurs in at least one plane.

In a further embodiment of this invention, in a vehicle, including a right side wheel, driven by a right side hydraulic motor, and a left side wheel, driven by a left side hydraulic motor, any one of the previously defined dual piston pump units is in combination with a prime mover. The prime mover is operatively connected with the input portion of one of the first and second through shafts for driving the first and second through shafts for consequently driving the first and second main pumps. The first pump unit is operatively connected with one of the right and left side hydraulic motors and, the second pump unit is operatively interconnected with the other of the right and left side hydraulic motors, thereby permitting independent propulsion of the right and left side wheels.

Another version of the previously defined dual piston pump units further includes a hydraulic fluid filter in operative combination with the center block portion with this fluid filter being of the spin-on cartridge type.

One type of vehicle wherein the previously defined dual piston pump units find an application are zero turn radius mowing machines.

The previously-described advantages and features, as well as other advantages and features, will become more readily apparent from the detailed description of the preferred embodiments that follow.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of a single, variable displacement, hydraulic piston pump unit having an integral fluid reservoir;

FIG. 2 is a longitudinal, vertical, central section of the pump unit of FIG. 1;

FIG. 3 is a hydraulic circuit schematic for a single, variable displacement hydraulic pump unit having an integral reservoir;

FIG. 4 is a top plan view of an inline dual, variable displacement, hydraulic piston pump unit having an integral fluid reservoir;

FIG. 5 is a bottom plan view of the pump unit of FIG. 4;

FIG. 6 is a view of one side of the pump unit of FIG. 4;

FIG. 7 is a view of the other side of the pump unit of FIG. 4;

FIG. 8 is a view of one end of the pump unit of FIG. 4;

FIG. 9 is a view of the other end of the pump unit of FIG. 4;

FIG. 10 is a vertical sectional view, taken along line 10-10 of FIG. 9;

FIG. 11 is a hydraulic circuit schematic for a dual, variable displacement hydraulic pump unit having an integral reservoir;

FIG. 12 is a top plan view, partly in section, of the pump unit of FIG. 2, combined with a drive unit, such as, e.g., a rotatable electric motor, which together define a portable power unit; and

FIG. 13 is a schematic view, partly in section, of a drive mechanism for an application wherein first and second pumps of an inline, dual variable displacement pump unit independently drive, via separate hydraulic motors, the left side and right side wheels, respectively, of a vehicle.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the several drawings, FIGS. 1 and 2 illustrate a single, variable displacement hydraulic piston pump unit 20 having an integral fluid reservoir 24. FIG. 3 is a hydraulic circuit schematic for pump unit 20, which can take the form of an over-center axial piston pump (shown), a bent-axis piston pump, or a radial piston pump (neither shown), for example. A center through shaft 26 (FIG. 2) having an input end 27 for a main pump 22 of pump unit 20, is driven by a prime mover (FIG. 12), such as, for example, an internal combustion engine or an electric motor, at a predetermined, speed, in a predetermined direction. Although the size and number of pistons are fixed, changing the piston stroke length can change the displacement of the pump, with the stroke length being determined by the angle of the swashplate of main pump 22, in a manner well known in the art, by any corresponding stroke controlling device, for example, as per trunnion shaft 28 (FIG. 1). Trunnion shaft 28, in turn, is connected to a control handle (not shown) through a linkage installed in the machine. When an operator pushes the control handle in one direction, in terms, of operation, best seen in FIG. 3, main pump 22 delivers high pressure fluid to one of circuit lines 32 and 34 of a closed loop circuit (not shown per se). Changing the displacement of main pump 22 will change output fluid flow rate. Moving the swashplate or yoke of main pump 22 over-center will reverse the flow out of main pump 22. Thus, depending upon the direction of the over-center movement of the noted pump swashplate or yoke, line 32 or line 34 can be a high pressure supply line or a low pressure return line, again in a manner well known in the art.

The hydraulic circuit schematic of FIG. 3, for the purpose of illustration, also shows the addition of a hydraulic motor 36, such as a fixed displacement motor, connected to pump unit 20 via lines 32 and 34. A charge pump 38, driven via center through shaft other end 29 (FIG. 2), remote from input shaft end 27, supplies additional hydraulic fluid from integral internal reservoir 24, within sump housing 40 of main pump 22. As shown in FIG. 3, the fluid passes through a filer 64 and flows into the closed-loop circuit via a conduit line 43 by way of two combination check and system relief valves 46 associated with circuit lines 32 and 34, respectively. This serves to compensate for any possible fluid flow loss due to internal system leakage. A charge pump relief valve 46 is used to provide a relief path to integral reservoir 24. Closed loop circuit also includes a bypass valve 48, positioned between lines 32 and 34, in order to transfer fluid from one line to the other. In addition, a cooling orifice 50 is interposed, in conduit line 43, adjacent to main pump 22, with the latter also being in fluid connection with integral reservoir 24 via a supply line 52.

As best seen in FIGS. 1 and 2, pump unit 20, in addition to main pump 22, integral reservoir 24 in sump housing 40, and charge pump 38, also includes a cooling fan 54 that is connected with center through shaft 26 at output end 29 remote from input end 27. Furthermore, sump housing 40, in the region above integral sump 24, is provided with an air venting device, such as a breather cap 56 having a strainer 57, and a plurality of external, outwardly directed and elongated cooling fins 58, as well as a case drain port 67 and a case port drain plug 68. Further yet, pump unit 20 includes a central block portion 60, preferably having another plurality of external, outwardly directed and elongated cooling fins 61, with block portion 60 being located intermediate sump housing 40 and charge pump 38. Block portion 60 includes a preferably cylindrical internal recess portion 62 (FIG. 2) adapted for receiving a replaceable, preferably internal fluid filter unit 64, such as of the cartridge type, supported in a filter cap 66, which in turn is adapted to be operatively and sealingly interconnected with the peripheral inner wall of recess portion 62. Block portion 60 further includes a pair of reversible or alternate inlet/outlet ports 76, 78 that can function as either high fluid pressure outlet or low fluid pressure inlet ports for lines 32 and 34 in a manner well known in the art.

Turning now to FIG. 12, illustrated therein is the pump unit 20 of FIGS. 1 and 2 operatively interconnected, via its center through shaft 26, with a prime mover 70, such as a rotatable electric motor including electronic control boxes 72 and cables 74, to produce a portable hydraulic power-producing unit 75.

Continuing on, FIGS. 4-10 illustrate various views of an inline dual, variable displacement, hydraulic pump unit 80, also having an integral fluid reservoir 84 (FIG. 10). FIG. 11 illustrates a hydraulic circuit schematic for this dual pump unit. Specifically, dual pump unit 80, other than using dual pumps shown, for example, as arranged in a back-to-back configuration in FIGS. 4-10, is quite similar both the structure and function of single pump unit 20 of FIGS. 1 and 2, with like parts thus being identified with like numerals and the addition of the suffixes “a” and “b” to distinguish the two pump units. Thus, dual pump unit 80 is comprised of first and second single pump units 20 a, 20 b, each very similar to previously described pump unit 20, but sharing a common center block or joinder portion 86. The inner ends 92 and 94 of the respective pump housings 88 and 90 of pump units 20 a, 20 b are attached to opposite axial end surfaces of center block portion 86, such as via a plurality of nuts and bolts 82, 84. In addition, the preferably splined inner ends 96 a, 96 b of center through shafts 26 a, 26 b are coaxially coupled within center block portion 86 via a preferably complementarily splined sleeve coupling member 98. The outer end 29 a of shaft 26 b serves to drive the single charge pump 38 as well as the cooling fan 54 of dual pump unit 80. In addition, only a single case drain port plug 68, a single breather cap 56, and a single, preferably internal filter unit 64, with cap 66 needs to be utilized.

For the purpose of illustration, the hydraulic circuit schematic of FIG. 11 also shows the addition of a second hydraulic motor 42 that is driven by second pump unit 20 b in the same manner as previously described relative to motor 36 driven by single pump unit 20. In terms of operation, dual pump unit 80 operates basically in the same manner as single pump unit 20 except that since single pump units 20 a and 20 b, which comprise dual pump unit 80, are coupled together, thus requiring but one input via shaft end 27 a. Single charge pump 38 is coupled to both pump units 20 a, 20 b via conduit lines 43 a and 43 b, respectively. Even though there are two pumps 20 a, 20 b, only a single integral hydraulic fluid reservoir 84 is required, together with but one case drain port plug 68, one filter 64, one breather 56, and one cooling fan 54.

Turning finally to FIG. 13, illustrated therein, partly in section, is a drive mechanism 100 for a vehicle application 102 that requires the independent operation of the left side wheel 104 and the right side wheel 106. Drive mechanism 100 can take the form of dual, variable displacement pump unit 80, wherein first pump unit 20 a is coupled with left side wheel 104 and its hydraulic motor 36 via lines 32 a, 34 a while second pump unit 20 b is coupled with right side wheel 106 and its hydraulic motor 42 via lines 32 b, 34 b.

It should be understood that while dual pump unit 80 is illustrated as an inline or tandem arrangement in FIGS. 4-10, this invention is not limited to thereto and can also include parallel or side-by-side configurations or structures that utilize but a single integral hydraulic fluid reservoir in a common sump housing. Such configurations can also utilize single cooling fans, charge pumps, oil filters, vent plugs, and drain port plugs. In addition, other configurations, utilizing at least a common integral hydraulic reservoir, such as angled arrangements in common or intersecting planes are also feasible. Furthermore, while internal fluid filters are preferred in order to keep overall package sizing to a minimum, external or spin-on type fluid filters can also be utilized.

It should also be clear, with particular reference to FIG. 13, that the inline dual piston pump unit 100 illustrated therein is particularly well suited for ZTR mower applications in that the unit is tall in the vertical direction and narrow in the horizontal direction, with this shape requiring less space in an inline installation.

It is deemed that one of ordinary skill in the art will readily recognize that the several embodiments of the present invention fill remaining needs in this art and will be able to affect various changes, substitutions or equivalents and various other aspects of the invention as described herein. Thus, it is intended that the protection granted hereon be limited only by the scope of the appended claims and their equivalents. 

1. A variable displacement hydraulic piston pump unit comprising in combination: a. one of an over-center axial piston main pump, a bent axis piston main pump, and a radial piston main pump; b. said main pump being rotatably journalled within a pump unit housing, said housing being of a size sufficient to include an internal volume portion that serves as an integral fluid reservoir for said pump unit.
 2. The piston pump unit of claim 1, wherein said pump unit housing further includes an air venting device.
 3. The piston pump unit of claim 2, wherein said fluid reservoir includes a case drain port.
 4. The piston pump unit of claim 1, wherein said fluid reservoir includes a case drain port.
 5. The piston pump unit of claim 1 further including a block portion operatively interconnected with and having one end thereof physically adjoining one end of said pump unit housing, said block portion further including a pair of reversible hydraulic fluid inlet and outlet ports.
 6. The piston pump unit of claim 5, wherein said pump unit housing further includes an air venting device.
 7. The piston pump unit of claim 5, wherein said fluid reservoir includes a case drain port.
 8. The piston pump unit of claim 7, wherein said pump unit housing further includes an air venting device.
 9. The piston pump unit of claim 5, wherein said block portion includes an internal recess portion.
 10. The piston pump of claim 9 further including a hydraulic fluid filter, said filter being located within said center block internal recess portion.
 11. The piston pump unit of claim 5 further including an axial center through shaft, said through shaft extending axially through said main pump housing, said main pump and said block portion, said through shaft including an input portion, extending from another end of said pump unit housing remote from said block portion, said through shaft further including another portion, extending from another end of said block portion remote from said pump housing, with said through shaft driving said main pump.
 12. The piston pump unit of claim 11 further including a charge pump, said charge pump having one end thereof adjoining said another end of said block portion, said charge pump being operatively interconnected with said block portion and driven by said another portion of said through shaft.
 13. The piston pump unit of claim 12 further including a cooling fan, said cooling fan being axially spaced from another end of said charge pump, remote from said block portion, and driven by said another portion of said through shaft.
 14. The piston pump unit of claim 11 further including a cooling fan, said cooling fan being axially spaced from said another end of said block portion and driven by said another portion of said through shaft.
 15. The piston pump unit of claim 11 in combination with a prime mover, said prime mover being operatively connected with said input portion of said through shaft for driving said through shaft and consequently said main pump; said pump unit, in combination with said prime mover, comprising a portable hydraulic power-producing unit.
 16. The piston pump unit of claim 12 in combination with a prime mover, the latter being operatively interconnected with said through shaft for driving said main pump and said charge pump; said pump unit, in combination with said prime mover, comprising a portable hydraulic power-producing unit.
 17. The piston pump unit of claim 13 in combination with a prime mover, the latter being operatively interconnected with said input portion of said through shaft for driving said main pump, said charge pump and said cooling fan; said pump unit, in combination with said prime mover, comprising a portable hydraulic power-producing unit.
 18. The piston pump unit of claim 5 further including a hydraulic fluid filter in operative combination with said block portion.
 19. The piston pump unit of claim 18, wherein said hydraulic fluid filter is of the spin-on cartridge type.
 20. A dual variable displacement hydraulic piston pump unit comprising in combination: a. a first pump unit including a variable displacement axial piston main pump rotatably journalled within a first pump unit housing, said first main pump adjoining b. a second pump unit including a variable displacement axial piston pump rotatably journalled within a second pump unit housing; c. said first and second pump unit housings being operatively interconnected and of a size having an internal volume portion sufficient to serve as an integral hydraulic fluid reservoir for said first and second pump units.
 21. The dual piston pump unit of claim 20, wherein at least one of said pump unit housings includes an air venting device.
 22. The dual piston pump unit of claim 21, wherein said fluid reservoir includes a case drain port.
 23. The dual piston pump unit of claim 20, wherein said fluid reservoir includes a case drain port.
 24. The dual piston pump unit of claim 20 further including a center block portion operatively interconnected, on each opposed axial end, with an adjoining first end of each of said first and second pump unit housings, said center block portion including a pair of independent reversible hydraulic fluid inlet and outlet ports for each of said first and second pump units.
 25. The dual piston pump unit of claim 24, wherein at least one of said pump unit housings includes an air venting device.
 26. The dual piston pump unit of claim 24, wherein said fluid reservoir includes a case drain port.
 27. The dual piston pump unit of claim 26, wherein at least one of said pump unit housings includes an air venting device.
 28. The dual piston pump unit of claim 24, wherein said center block portion includes an internal recess portion.
 29. The dual piston pump unit of claim 28 further including a hydraulic fluid filter, said filter being located within said center block internal recess portion.
 30. The dual piston pump unit of claim 24, wherein said first and second pump units further include a respective first and second axial center through shaft, said first and second through shafts extending axially through their respective main pump housing, main pump and block portion; one of said first and second through shafts including an input portion, extending from a first another side of its respective pump unit housing remote from said center block portion; another of said first and second through shafts including another portion, extending from a second another side of its respective pump housing remote from said center block portion, with said first and second through shafts driving their respective main pumps.
 31. The dual piston pump unit of claim 30, wherein each of said first and second axial center through shafts further includes an inner end portion, said inner end portions extending into and operatively interconnecting with each other within said center block portion.
 32. The dual piston pump unit of claim 30 further including a charge pump, said charge pump having one end adjoining said second another side of said respective pump housing, said charge pump being driven by said another portion of one of said first and second through shafts.
 33. The dual piston pump unit of claim 32 further including a cooling fan, said cooling fan being axially spaced from another end of said charge pump, said cooling fan being driven by said another portion of one of said first and second through shafts.
 34. The dual piston pump unit of claim 20, wherein said first and second pump units are arranged in an inline configuration.
 35. The dual piston pump unit of claim 34, wherein said inline configuration is in a back-to-back arrangement.
 36. The dual piston pump unit of claim 24, wherein said first and second pump units are arranged in an inline back-to-back configuration, with said center block portion being interposed therebetween.
 37. The dual piston pump unit of claim 20, wherein said first and second pump units are arranged in a side-by-side configuration.
 38. The dual piston pump unit of claim 20, wherein said first and second pump units are angularly offset relative to each other.
 39. The dual piston pump unit of claim 38, wherein said angular offset occurs in at least one plane.
 40. In a vehicle, including a right side wheel, driven by a right side hydraulic motor, and a left side wheel, driven by a left side hydraulic motor, the dual piston pump unit of claim 31, in combination with a prime mover, said prime mover being operatively connected with said input portion of one of said first and second through shafts for driving said first and second through shafts for consequently driving said first and second main pumps; said first pump unit being operatively connected with one of said right and left side hydraulic motors and said second pump unit being operatively interconnected with the other of said right and left side hydraulic motors, thereby permitting independent propulsion of said right and left side wheels.
 41. In a vehicle, including a right side wheel, powered by a right side hydraulic motor, and a left side wheel, powered by a left side hydraulic motor, the dual pump unit of claim 32, in combination with a prime mover, said prime mover being operatively connected with said input portion of one of said first and second through shafts for driving said first and second main pumps and said charge pump; said first pump unit being operatively connected with one of said right and left side hydraulic motors and said second pump unit being operatively connected with the other of said right and left hydraulic motors, thereby permitting independent rotational movement of said right and left side wheels.
 42. In a vehicle, including a right side wheel, rotatable via a right side hydraulic motor, and a left side wheel, rotatable via a left side hydraulic motor, the dual pump unit of claim 33, in combination with a prime mover, the latter being operatively interconnected with said input portion of one of said first and second through shafts for driving said first and second main pumps together with said charge pump and said cooling fan; said first pump unit being operatively connected with one of said right and left side hydraulic motors and said second pump being operatively connected with the other of said right and left hydraulic motors, thereby permitting independent angular movement of said right and left side wheels.
 43. The dual piston pump unit of claim 42 further including a hydraulic fluid filter in operative combination with said center block portion.
 44. The dual piston pump unit of claim 43, wherein said hydraulic fluid filter is of the spin-on cartridge type.
 45. The vehicle of claim 42, wherein said vehicle is a zero turn radius mowing machine. 